1. Field of the Invention
The present invention relates to optical memory systems using anamorphic prisms for light-beam shaping and/or steering. More particularly, the present invention involves minimizing errors and beam angle changes which result from lateral chromatic aberration in the anamorphic prism. The invention also involves providing multiple surfaces to reflect the return beam to various detectors. The invention has particular advantages in optical disk systems, for instance, optical data storage systems, compact disc systems, and laser video disk systems.
2. Description of the Related Art
Optical disk systems often employ an anamorphic prism for adjustment of laser beam ellipticity, for the removal of laser beam astigmatism, and/or for beam steering. References such as U.S. Pat. No. 4,333,173 issued to Yonezawa, et al., U.S. Pat. No. 4,542,492 issued to Leterme, et al. and U.S. Pat. No. 4,607,356 issued to Bricot, et al. describe using simple anamorphic prisms for beam shaping in optical disk applications.
Frequently, the anamorphic prism systems have an embedded thin film to reflect some or all of a returning beam (reflected from optical media) to a detection system. U.S. Pat. No. 4,573,149 to Deguchi, et al describes the use of thin films to reflect a return beam to detection systems. Furthermore, the entrance face of the anamorphic prism is often used to reflect the returning beam to a detection system as described in U.S. Pat. Nos. 4,542,492 and 4,607,356. Often, it is advantageous to have multiple detection channels. For instance, in optical disks, one detector may provide data signals and another detector may provide control signals such as tracking and/or focus servo signals.
A typical problem with conventional prisms is that the anamorphic prism suffers from chromatic dispersion which can result in lateral chromatic aberration. In other words, when the wavelength of the light source changes, the resulting angles of refraction through the anamorphic prism also change. These changes result in a lateral beam shift when the beam is focussed onto optical media such as an optical disk. In optical disk systems, a small shift in the beam may cause erroneous data signals. For instance, if the shift is sudden and in the data direction, the beam may skip data recorded on the optical disk.
If the light source (e.g., a laser) were truly monochromatic, the chromatic aberration in the prism would not cause a problem. However, several factors often cause the laser spectrum to change. For instance, most laser diodes respond with a change in wavelength when the power increases. In magneto-optic disk systems, an increase of power occurs when pulsing the laser from low to high power to write to the optical disk, as is well understood in the art. This increase in laser power often causes a wavelength shift of around 1.5 to 3 nanometers (nm) in conventional systems. Most laser diodes also respond to a change in temperature with a change in the wavelength. Additionally, random "mode-hopping" can cause unpredictable wavelength changes commonly ranging from 1-2 nanometers. RF modulation is often applied to laser diodes operating at read power in order to minimize the effect that "mode-hopping" has on the system. However, the RF modulation increases the spectral bandwidth and can change the center frequency. Moreover, RF modulation is not generally used when the laser is operating at write power. In a non-achromatic system, a sudden change in the wavelength of the incident light typically results in a lateral beam shift in the focussed spot of up to several hundred nanometers. A lateral beam shift of this magnitude could cause significant errors in the data signal.
Using multi-element prism systems to correct chromatic dispersion is known in the art of optical design. Textbooks such as Warren J. Smith, Modern Optical Engineering, McGraw-Hill, 1966, pp. 75-77 discuss this idea. Furthermore, some optical disk systems use multi-element anamorphic prism systems which are achromatic. However, typical existing multi-element prism systems require the multiple prism elements to be separately mounted. Mounting the multiple elements increases the expense and difficulty of manufacturing because each element must be carefully aligned with respect to the other elements in the system. Small deviations in alignment can cause significant variations in function. This also complicates quality control. Other existing multi-element prism systems have attached elements to form a unitary prism, but these prism systems require that the prism material of each prism be different in order for the system to be achromatic. Finally, existing systems which are achromatic do not provide return beam reflections to multiple detection systems.